1. Field of the Invention
The present invention is in the field of power converters. The present invention is further in the field of semiconductor switching power converters. The present invention further relates to the field of integrated synthetic ripple control methods for switching power converters and circuits. The present invention is further in the field of integrated switching power converters. The present invention is further in the field of hysteretic control types for switching power converters. The present invention is further in the field of multi-phase switching power converters. The implementation is not limited to a specific technology, and applies to either the invention as an individual component or to inclusion of the present invention within larger systems which may be combined into larger integrated circuits.
2. Brief Description of Related Art
Modern electronic applications require power management devices that supply power to integrated circuits or more generally to complex loads. In general, power switching converters are becoming more and more important for their compact size, cost and efficiency. Switching power converters comprise isolated and non isolated topologies. The galvanic isolation is generally provided by the utilization of transformers. Although the subject invention is mainly focused on non isolated switching power converters, it may refer in part to isolated power converters as well.
Modern switching power converters are in general divided in step down power converters, also commonly known as “buck converters”, and step up power converters commonly known as “boost converters”. This definition stems from the ability of the converter to generate regulated output voltages that are lower or higher than the input voltage regardless of the load applied.
One class of modern switching power converters implemented in integrated circuits is the one comprising hysteretic control or pseudo-hysteretic control where a synthetic ripple signal is generated and compared to a reference to determine the duty cycle of the switching period in order to regulate the output voltage at the desired level. These hysteretic power converters do not include an error amplifier, a specific compensation network or a periodic signal to determine the switching frequency.
In fact their switching frequency is determined by several factors like the input voltage, the output voltage, the load, the output capacitor value, the inductor value, the hysteresis value, and the general propagation delays of the feedback network, of the comparator, of the driver, and of the output stage. However frequency control circuits are commonly implemented in order to control the frequency.
For high load current applications it is not uncommon to use multi-phase converters which are viewed as a number of power converters to effectively operate in parallel in order to provide higher currents. The advantages of multi-phase switching converters are their higher power density, their lower current ripple for the same output capacitor, their superior transient response and their cost effectiveness for high current loads. Furthermore the efficiency of the power conversion is better optimized over the full range of loads because at light loads some phases of the converters may be completely turned off to reduce the switching losses.
The phases of the converter have to be switched out of phase with respect to each other in order to reduce the supply and ground voltage ripple and noise. This can be obtained in several ways, but the greatest simplification comes from having fixed frequency types of control because the misalignment of the switching phases is more easily obtained. Although the hysteretic type of control offers several advantages in terms of load transient response, the control of the switching frequency is not always the easiest.
Therefore since the fixed frequency ripple switching converters offer similar responsiveness, bandwidth, fast switching frequency and they operate at known and fixed frequency they are very good candidates for multi-phase operation. One of the problems of the multi-phase switching power converters is that the load current tends to spread unevenly in the phases especially when the load current is high. This represents a major issue because the advantages of multi-phase converters vanish if most of the current flows in one or two phases and, in addition, reliability problems arise if currents higher than expected flow for long times in one phase.
The reasons for the current unbalance are the unavoidable mismatch in the main inductors' series resistance, the mismatch of the on-resistance of the power devices, and errors and offsets in computing the duty cycle for each phase. Many configurations for controlling the multi-phase power converter exist and the main two configurations involve either having one controller for all the phases or many individual controllers, where each one controls one phase. The case of individual controllers, although only apparently more onerous, eliminates the possible error due to the duty cycle computing.
Many techniques to achieve current balance have been explored in the past and documented in prior art documents, however all include a first phase of sensing the current in each phase and subsequently a second phase of correcting the duty cycle to equalize the currents. The current sensing is mainly performed by either placing a small resistor in series to the inductor or by sensing the voltage drop on one of the power transistors when it is on. The first approach is more expensive and less efficient but more accurate.
FIG. 1 depicts a typical prior art simplified block diagram of a four-phase buck converter with current sensing done by monitoring the voltage drop across series resistances. The sensing voltage at the nodes between the inductor and the resistor for each phase is fed to the power converter controller 1. The block 1 controls the synchronization of the switching for each phase, computes the current sensing information to determine the duty cycle correction for each phase and in general includes the output voltage control loop for regulation. The power converter controller feeds the driver blocks 2 with the driving signal to operate the power train.
The prior art Zane et al (U.S. Pat. No. 7,479,772) is an example of how complex current sharing can be for a multiphase converter. The current sensing information is passed to a controller to adjust the duty cycle of each phase maintaining the overall regulation of the output voltage and controlling the misalignment of the phases. In this case the information is digitalized adding to the complexity and limiting the bandwidth of the regulator. Wilcox et al (U.S. Pat. No. 5,847,554) is one of many examples of sensing the current for current sharing by checking the voltage drop of both power transistors when they are on, or the voltage drop of the low side power transistor during recirculation of the current.
The prior art patents Walters et al (U.S. Pat. No. 5,982,160) and Lethellier (U.S. Pat. No. 6,441,597) represent two examples of current sensing by measuring the voltage across the main capacitor of the feedback network or as a result of an RC time constant across the inductor, but if the series resistance of the inductor (ESR) is mismatched between phases, the current sensing through the RC is highly inaccurate. However for the cited cases of Walters and Lethellier the proposed technique does not make reference to multi-phase power converters.
Similarly Shuellein et al (U.S. Pat. No. 7,301,314) reports the same method for sensing the current in the phases of a multi-phase converter, but the same considerations of inaccuracies in presence of ESL mismatch apply. A clever technique is described in Sato et al (U.S. Pat. No. 7,466,116) where the current sensing is achieved by sensing the voltage drop across the bonding wire. This is quite interesting but today, in many applications (especially high frequency), the bonding wire cannot be used for their excessive intrinsic parasitic inductance and resistance. Furthermore the difficulty in checking the differential voltage may arise from the difficulty in having a good Kelvin connection of the true ground signal.
More interesting appears the approach taken by Skelton et al (U.S. Pat. No. 6,160,388). Skelton teaches a method to sense the current by monitoring the voltage drop across the low side power transistor when turned on and to hold the information into a capacitor. However the proposed circuitry to obtain this involves the utilization of operation amplifiers and switches limiting the bandwidth of the sensing and adding to the complexity of the system. It is worth mentioning that this was not intended for multi-phase current sharing.
An example of prior art of current balancing for multi-phase power converter is shown in Mattingly (U.S. Pat. No. 7,301,317) where an RC network in the multi-phase step down power converter is configured to program to desired power distribution between the phases. However, as mentioned above, this method is highly inaccurate and costly.
Another example of current balancing for multi phase power converters is described in Groom et al (U.S. Pat. No. 6,495,995). Groom teaches a method for sensing the current of a ripple multi-phase converter by using a series sense resistor and by trimming the duty cycle depending on an output of differential amplifier that senses one slave phase current with respect to a master signal. The unbalance also causes a differential voltage droop of the output that is corrected by the virtual ripple generator block. There is only one virtual ripple generation block for the whole power converter.
All the cited prior art does not describe a cost effective, accurate and simple method for achieving active current sharing in multi-phase switching power converters with optimum transient performance maintaining the system stability in all conditions. It is therefore a purpose of the present invention to describe a novel multi-phase switching power converter that combines the characteristic of simple adaptive adjusting of the duty cycle of each phase in order to obtain continuous current balance between the phases.
It is another purpose of the present invention to describe a power converter that can respond to abrupt and large load transients by forcing maximum duty cycle until the output current is equal or slightly higher than the load current so as to minimize the output voltage droop and recover from it in the shortest time possible. It is another purpose of the present invention to describe a constant frequency multi-phase ripple power converter that is stable and can operate at high switching frequencies.